1. Field of the Invention
The present invention relates to a process for preparing high quality titanium dioxide which may be in the anatase or rutile crystal form.
2. Description of the Prior Art
The preparation of titanium dioxide has been accomplished via various techniques. In one well known process commonly referred to as the "Blumenfeld Process" (see U.S. Pat. No. 1,795,467), a titanium sulfate solution prepared by the action of sulfuric acid on titanium bearing ores, such as ilmenite or rutile, is heated and the solution is mixed with a quantity of hot water. A precipitate of hydrous titanium oxide immediately forms, but dissolves on continued addition of the titanium sulfate solution. Upon continued heating, the hydrous titanium oxide again begins to precipitate and after several hours it is completed whereby a yield of around 95% may be obtained.
In another well established process described in U.S. Pat. No. 1,758,528, a small amount of a titanium salt solution which has been neutralized with an alkaline agent to a pH within the range of 4.0 to 4.5 is added as a seed suspension to a titanium salt solution and the mixture is heated. According to the patent, the process results in a high yield (e.g., about 95%) of titanium dioxide hydrate.
In U.S. Pat. No. 2,029,881, hydrolysis of titanium salt solutions is hastened by the addition of a dry alkali metal titanate.
More recently, high quality titanium dioxide has been prepared by more specialized hydrolysis procedures. For example, in U.S. Pat. No. 3,615,204, a measured quantity of a defined clarified, uncrystallized and unconcentrated titanium sulfate-ferrous sulfate solution is added to an empty precipitation tank. A nucleating agent containing 12 to 16% solids, which solids are composed of 85 to 89% TiO.sub.2 and 11 to 15% Na.sub.2 O at a mole ratio of TiO.sub.2 :Na.sub.2 O in the range of from 5.7 to 8.1:1.0, is added to the solution under agitation. The mixture is then boiled, filtered, bleached, slurried with water, treated with defined potassium and phosphorus salts, calcined and milled.
In another procedure, as described in U.S. Pat. No. 3,617,217, a measured quantity of a defined clarified, uncrystallized and unconcentrated titanium sulfate-ferrous sulfate solution is added to hot agitated water at defined intervals and at defined temperatures. The water is present in an amount ranging from 5 to 20% by volume of the measured quantity of solution. The final mixture is then boiled, filtered, bleached, slurried with water, treated with defined potassium and phosphorus salts, calcined and milled.
In an effort to produce high yields of high quality titanium dioxide, the art has tried to vary a vast array of operating parameters on what can best be described as a trial and error method. While occasionally resulting in acceptable results, there has been no assurance that essentially the best results have been obtained. This is particularly true when the quality or source of the titanium dioxide changes from time to time.